1. Field of the Invention
This invention is related to closed circuit television systems in general and a system for synchronization of remotely located cameras in particular.
2. Description of the Prior Art
Closed circuit television (CCTV) systems, particularly those used for security or surveillance, generally include a number of television cameras mounted remotely from a central monitoring station. Often these cameras are located at substantial distances from the monitoring station. For example, distances of a mile or greater are not uncommon.
The signal in NTSC television, which is the standard used in most broadcast and CCTV systems in the United States, generally contains thirty frames per second, in which each frame includes 2 fields of 262 1/2 lines, or a total of sixty fields per second. Two fields are interleaved to yield a frame of 525 lines every thirtieth of a second. Each field or frame starts from the upper left hand corner of the camera's field of view, and each line begins at the left hand side of the field of view.
The video signal is most often generated by a video or television camera and received by a television or video monitor. In order for the picture information to be intelligibly reproduced by the monitor it is necessary that the cathode ray tube (CRT) in the monitor trace out or "paint" the picture from the camera on the CRT in synchronization with the generation of the picture at the video camera. Thus it is necessary that the monitor begin each frame and field at the upper left hand corner of the CRT and each successive line at the left hand side of the CRT screen in synchronism with the signal generated by the camera. In order to accomplish this synchronization, two synchronization or "sync" signals are required, namely a vertical and a horizontal sync signal.
The vertical sync signal is used to cause each frame to begin at the upper left hand corner of the CRT. Since there are sixty fields per second, and thirty frames per second this vertical synchronization signal occurs the rate of 30 hz.
The horizontal sync signal is used to cause each line painted on the CRT to begin at the left hand side of the screen. Since there are a total of 525 lines scanned thirty times per second, this takes place at a rate of 15,750 hertz.
A closed circuit television system used, for example, for security purposes, generally includes more than one camera. An operator sitting at the console ordinarily needs to be able to see what each and every camera "sees." While it is possible to have a separate monitor dedicated to each camera, it is generally far more cost effective to have a single monitor that successively receives a signal from several remote cameras and displays this information in a predetermined sequence on the monitor screen. Unless the cameras are synchronized to each other, each camera will essentially send signals having a random timing and thus the picture on the monitor will roll until it has synchronized with the next camera in the sequence. Not only is this rolling extremely distracting to a person viewing the monitors, it ultimately limits the speed at which switching between cameras takes place. This can be a particular problem in high speed switching circuits, or digital recording circuits. Such circuits record one or two frames from successive cameras on a VCR or other video tape recorder and switch between sources in as little as 32 ms.
Problems are also encountered if the operator wishes to use a split screen display. That is, one in which the left hand side of the television screen contains information from one camera and the right hand side of the screen contains picture information from another camera.
To overcome this problem, in the prior art all remote cameras are synchronized with the sixty hertz power line frequency. Since this is double the number of frames per second, and equal to the numbers of fields per second, it is often sufficient for low speed systems. Reliance on the power line frequency can only provide synchronization of the vertical sync signals, however, and not the horizontal sync signals. Furthermore, any cameras not located near the same sixty hertz power source, cannot take advantage of synchronizing to the AC power line. Thus a battery operated camera would not be in synchronism with the other cameras.
In order to overcome these problems, that is provide synchronization with even battery operated cameras, as well as provide synchronization at the horizontal sync rate, a single master synchronization reference can be generated either by one master camera (or other video sources) or the central station containing the monitors. This signal may be transferred to all the other cameras in the system. This transfer is generally performed over a separate set of wires dedicated to carrying the sync signals. In many installations, the cost of the additional wiring is prohibitive, particularly if there are numerous cameras situated at large distances from the central station.
It is thus an object of the present invention to provide a method and apparatus for permitting the synchronization of horizontal and vertical sync signals from several remotely spaced cameras.
It is a further object of the invention to permit synchronization of the sync signals from all cameras without the need for extra wiring devoted to the carrying of a master sync signal.
These and other objects, advantages and features of the invention will be more apparent upon reference to the following specification and the annexed drawings.